[postgis-users] drawing nautical data on projection 4326[solution?]

[Morin, Marc-André]

Hi Dave,
 
You are certainly not on the right track, because the SRS EPSG:4326 is not a projection system, but more a datum (WGS84), based on an ellipsoid (GRS80).  In a nutshell, the 4326 srs is a "spherical" geographic system and that is why the unit of measurement is in degree.  So when you store your data in EPSG:4326, that means your data will have to be stored in latitude and longitude, and you cannot represent lat-long coordinates directly on a map, unless these coordinates are projected in a cartesian (X and Y) coordinate system, like the Google Mercator projection (EPSG:900913).
 
That said, you specified in your first mail that your were working with "lines" measured in nautilcal miles, within a grid graduated for 0 to 100.  That means that you are already in a "projected" coordinate system.   Now, we have to figure out what is this coordinate system. Maybe I am wrong, but your 0-100 grid seems to represent a cartesian coordinate system in GRAD (not in degree)...  
 
Could you check and confirm that at first?  Also, a screenshot of your map would be helpful...
 
Regards,
 
Marc-André
 

________________________________

De : postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] De la part de Dave
Envoyé : 29 avril 2011 04:05
À : PostGIS Users Discussion
Objet : Re: [postgis-users] drawing nautical data on projection 4326[solution?]


I think I have a solution to this issue

I am going to convert my point origin to meter based projection (eg google 900913), create my line data, then transform it back to the 4326 projection,  in this way the scale factors calculations should be done by the proj library.

Does this sound like a decent solution?

On 28/04/2011 10:48, Dave wrote: 

	I have some line  data measured in nautical miles.
	
	I want to draw these points on a map using the 4326 projection, the srs entry for this projection gives the unit of measurement as degree.
	
	A nautical mile is one minture of an arc of lattitude ie 0-60, the projection is measured in 0-100.
	
	So does this mean that when I want to draw a line measured in nautical miles,  I have map 0-60 on to 0-100 grid , ie I have to apply  0.6 factor ?
	
	So a line  starting at lat 1.2 and ending at  lat 2.7 becomes a line 1.12  to  2.42 ?
	
	regards
	
	
	Dave.
	
	
	
	
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